OFDMA wireless communications generally support multiple simultaneous users by scheduling each user a subset of the OFDM frequency band at different points in time. In downlink, the base station (BS) typically sends reference signals distributed across the whole frequency band in every scheduling period, so the user equipment (UE) can do frequent channel estimation for the whole frequency band, even though it may not be scheduled to receive any data in that time period or may only be scheduled to receive downlink data in a small portion of that band. However, for OFDMA uplink transmissions, a UE generally only sends reference signals in the portion of the frequency band allocated for its data transmission. LTE provides a sounding reference signal (SRS) mechanism for UEs to send wideband reference signals to allow the BS to do channel estimation for the uplink channel from each UE.
Base stations with multiple antennas and knowledge of the channel state can use beamforming to improve system throughput or capacity in bit/s/Hz. The SRS channel estimates can be used in constructing the precoding matrix for downlink transmissions and can be used in constructing the detection matrix for uplink reception. It is required to periodically refresh the channel estimates, since the wireless channel changes over time. The channel state is considered to be only valid for providing beamforming gain in the precoding or detection matrix for the “coherence time period” and the beamforming gain achievable gradually decreases as time delay after channel estimation increases. The coherence time is different for each user within a specific wireless network and depends on UE speed of motion.
Massive MIMO systems increase system data throughput capacity by increasing the number of users simultaneously sharing the same time and frequency resources in both uplink and downlink data transmission periods. There is a need to correspondingly increase the system capacity for channel estimation from SRS transmissions, in terms of the number of users per SRS transmission and the rate of update per user relative to the coherence time. PCT patent application PCT/US15/56500 entitled “Methods for Channel Information Acquisition, Signal Detection and Transmission in Multi-User Wireless Communication Systems” filed on Oct. 20, 2105 (claiming the benefit of U.S. Provisional Application No. 62/065,775, filed on Oct. 20, 2014) has shown that optimal performance in a Massive MIMO system requires perfect knowledge of the following channel characteristics at the instant of applying the precoding or detection matrix; symbol time offset between each UE and the base station, carrier frequency offset between each UE and the base station and the CSI (channel state information) for the multipath fading channel between each UE and BS antenna. Moreover, in a practical system, there is a time delay between calculating these matrices and applying them, so it also follows that for optimal performance, the BS needs perfect knowledge of the UE speed, since this affects the coherence time.
This invention describes SRS transmission methods that are enhanced relative to current LTE Rel.12 SRS transmission methods to meet the Massive MIMO requirements described above. These enhancements are described in two separate categories. Firstly, enhancements to multiplex more channels in the same resources for a given SRS transmission and to provide Massive MIMO throughput performance improvements. Secondly, scheduling enhancements to provide adaptive UE-specific scheduling based on channel conditions for each UE. These methods can be combined in different ways in each BS to optimize the channel estimation system capacity for a given level of system performance.